U.S. Pat. No. 4,620,783 discloses an image-forming apparatus having a plurality of development units indexable into alignment with a development position associated with a photoconductive drum. A single bulk toner detector is provided for sensing the amount of toner in a toner supply of whichever development unit is aligned with the drum.
U.S. patent applications Ser. No. 632,677, filed Dec. 24, 1990 to Kroll et al and Ser. No. 770,266, filed Oct. 3, 1991 to Miller et al show a series of development units which move through a linear path, which path brings each station successively into alignment with a development position. A single toner monitor is positioned along the path to monitor developer in each station when positioned adjacent the monitor.
U.S. Pat. No. 5,111,245, issued May 5, 1992 to DeCecca et al and U.S. patent application Ser. No. 669,701, filed Mar. 15, 1991 to Kroll et al also show a linearly moving set of toning units in which each unit is moved into alignment with a development position and then only the applicator of each unit as it is aligned is moved into the toning position or toward the toning position from a position slightly spaced from the position.
U.S. patent application Ser. No. 794,761, filed Nov. 19, 1991 to DeCecca et al (a continuation of U.S. patent Ser. No. 632,706, filed Dec. 24, 1990) shows a rack and pinion mechanism for translating a series of development stations through a position aligned with a development position.
Present state of the art toner monitors have their greatest sensitivity and greatest accuracy if they are placed as close to the mixture being monitored as possible. For this reason, monitors are commonly placed in a strategic location inside a sump of a toning station. In situations in which a monitor is not part of the station itself, it is placed closely adjacent a wall of the station and the wall may be thinned at that location to improve sensitivity.
If a single monitor is to be used to sense toner concentration of more than one movable station with consistency and sensitivity, that monitor must be positioned repeatedly in close proximity to the mixture being sensed.